In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Magnesium is the eighth most abundant element in the earth's crust and essential to most living species. It can form several forms of hydrated carbonates such as nesquehonite (MgCO3.3H2O), and lansfordite (MgCO3.5H2O), a number of basic carbonates such as hydromagnesite (4MgCO3. Mg(OH)2.4H2O), and dypingite (4MgCO3. Mg(OH)2.5H2O) as well as the anhydrous and rarely encountered magnesite (MgCO3). The various forms of magnesium carbonate are all industrially important materials and for example used in pharmaceutics as antacids, adsorbents and diluents in direct compression tablets. They are also found in cosmetics thanks to their mild astringent properties which help to smoothen and soften skin, and have found their applications in dusting powders, face masks as well as in toothpastes. In addition, high purity magnesium carbonates are useful desiccants, for instance, as an additive in table salt to keep it free flowing or as a drying agent for hands to improve the grip, e.g. for rock climbing, gymnastics, and weight lifting.
Commercial (crystalline) analogues of magnesium carbonates typically show specific surface areas (SSAs) of about 4-18 m2 g−1. For previously reported X-ray amorphous magnesium carbonates produced by thermal decomposition of hydrated magnesium carbonates forms, the highest SSA found in the literature is ˜50 m2 g−1.
For many geologists, the anhydrous (native) magnesite is a conspicuous rock with unclear genesis. Although magnesium carbonates are abundant in nature in the form of minor traces in most geological structures, magnesium carbonate rarely exists as a monomineralic magnesite in economically viable deposits. In fact, there are virtually only two types of magnesite deposits in the world: the sparry magnesite of Vietsch type, which constitutes 90% of world's reserves and forms nearly monomineralic lenses within marine platform sediments, and the less common but highly valued Kraubath type magnesite of superior quality. The Kraubath type consists of veins (300-400 meter deep) and stockworks (80 meter deep) of cryptocrystalline “bone” magnesite, also sometimes referred to as gel-magnesite. It commonly occurs together with ultramafic rock structures such as serpentine ((Mg,Fe)3Si2O5(OH)4) and olivine ((Mg,Fe)2SiO4) minerals. The formation of Kraubath type magnesite is suggested to occur through a so-called epigenetic-hydrothermal route, wherein hydrothermal fluids of moderate temperature and low salinity carrying CO2 interact with ultramafic rocks. Most of the silica and iron derived from the decomposition of ultramafic rocks are carried to the surface whereas the veins of magnesite precipitate in situ as a gel.
In nature, magnesium carbonate occurs in two physical forms; as macrocrystalline or cryptocrystalline magnesite. The cryptocrystalline form is also sometimes referred to as amorphous or gel magnesite by geologists, however, it should be stressed that this does not imply that it is X-ray amorphous, merely that the size of the crystallites are too small to be observe with a light microscope. Hereinafter the term amorphous should be interpreted to mean X-ray amorphous.
X-ray amorphous magnesite has been observed upon thermal decomposition of crystalline hydrated magnesium carbonates occurring at temperatures of the order of 300° C. or higher. Such magnesites are, however, not stable upon long term storage in humid atmosphere as it has been shown that the carbonate bond is weakened during rehydration. This weakening is evident by the fact that the decarbonation peak in differential Thermogravimetic measurement (dTGA) curves at about 350° C. or above develops a shoulder and/or splits into two or more peaks and also shifts towards lower temperatures.
Interestingly, magnesite has stirred problems not only for the geologists but also for the chemists. Anhydrous MgCO3 can easily be produced at elevated temperatures. However, numerous authors, have described unsuccessful attempts to precipitate anhydrous magnesium carbonate from a magnesium bicarbonate solution kept at room temperature and under atmospheric pressure. Instead, hydrated magnesium carbonates or one of the more complex basic magnesium carbonates precipitated under such conditions leading to what has been branded as “the magnesite problem”.
In 1999, successful attempts of making crystalline magnesite at 400° C. and atmospheric pressure was presented by using a suspension of artificial sea-water with calcium carbonate and urea through which CO2 was bubbled followed by dissolution and titration with dilute ammonia during which carbonate precipitated. The precipitate was characterized as crystalline magnesite using X-ray diffraction, and traces of aragonite (CaCO3) and possibly calcite (CaCO3) were noted in the diffractogram. The experiment has since been repeated and the precipitates consisted of magnesite with traces of aragonite (CaCO3) and dypingite (Mg5(CO3)4(OH).5H2O). In both of the experiments magnesite was formed after 14 dissolution-precipitation cycles.
It should be mentioned that magnesium carbonate was attempted to be synthesized also in non-aqueous solvents during the early 1900's. However, it was concluded that magnesium carbonate cannot be obtained by passing CO2 gas through methanolic suspensions of MgO due to the more likely formation of magnesium dimethyl carbonate Mg(OCO)(OCH3)2.
Subsequent studies only reiterated the assumption that MgO preferentially forms complex dimethyl carbonates when reacted with CO2 in methanol. This conclusion was especially peculiar since carbonates of other rare earth metals, such as those of Ca, Ba, and Sr, can be readily produced by passing CO2 gas through alcoholic suspensions of their respective oxides.
In view of the above-mentioned industrial applications of magnesium carbonates and their non-toxic properties, further improvements in the magnesium carbonates and their production methods are desirable to allow for expanded use of magnesium carbonates in various applications. As well, introduction of a new class of magnesium carbonate containing materials with structural and functional properties that have currently not been found in previously disclosed magnesium carbonate containing materials are foreseen to open up for new industrial applications and for improved functionality in already existing applications. To become industrially attractive, areas of improvements include water sorption properties, porosity, specific surface area, long term stability of the material and the cost of production.
There are, to our knowledge, no prior art disclosing a magnesium carbonate material containing micro and/or meso pores, neither among the reports describing crystalline magnesium carbonates nor the X-ray amorphous ones produced by thermal decomposition. A nitrogen sorption analysis performed on, e.g., hydromagnesite (Mg5(CO3)4(OH)2.5H2O)), which is the pharmaceutical grade of magnesium carbonate, reveals a material with no porosity in the micro pore range and with some meso pores between, but not inside, the powder particles, as will become evident in the drawings and examples that follows.
Magnesium carbonates are well known for their desiccant properties in applications like those mentioned above, e.g., for keeping table salt free flowing in humid climates and as gripping agents in rock climbing. Existing magnesium carbonate majorly adsorbs moisture around or above 70% relative humidity (RH) at room temperature and are not know to be good moisture adsorbents at low RHs.
The stability of presently known amorphous and anhydrous magnesium carbonates, i.e. those produced by thermal decomposition of crystalline hydrated magnesium carbonates, are known to be limited upon storage in humid environments. The carbonate bond of such materials usually weakens after only 2 weeks of storage in 100% humidity, preventing a regeneration of the materials original structure and properties.